In recent years, there are growing demands for zoom lenses having large aperture ratios with tendencies toward the higher performance of digital cameras. The downsizing of an image-sensing optical system is accordingly required. As a zoom lens to fit with such a requirement, there is proposed an image-sensing optical system as described in JP-A-2005-331641.
The above image-sensing optical system has a first lens group with negative refractivity, a second lens group with positive refractivity and a third lens group with positive refractivity in the direction from an object side to an image side. The first lens group has a make-up of a lens with negative refractive power and a lens with positive refractive power, the second lens group has a make-up of a lens with positive refractive power and a lens with negative refractive power and the third lens group has a make-up of lenses with positive refractive power. The above image-sensing optical system has a member for determining a light reflux of an open F-number in a predetermined position.
When attention is focused on the above first lens group, the lens with negative refractivity (to be referred to as “lens G11” hereinafter) is formed of a glass having high refractivity represented by a refractive index (nd) of over 1.83 and a low dispersion property represented by an Abbe's number (νd) of 40 or more, and it is a concave meniscus lens both surfaces of which have aspherical forms. On the other hand, the lens with positive refractivity (to be referred to as “lens G12” hereinafter) is a spherical convex meniscus lens formed of a glass having high refractivity represented by a refractive index (nd) of 1.82 or more and a high dispersion property represented by an Abbe's number (νd) of 25 or less.